1. Field of the Invention
The present invention relates to a high impact polymer composition of excellent transparency and stress-whitening resistance characteristics. More particularly, this invention provides a resin composition which is excellently transparent, which has excellent gloss characteristics, which is not whitened by bending and which possesses excellent impact strength properties. The composition has a multi-layer structure comprising a rubber layer composed mainly of a diene monomer, hard inner and outer resin layers composed mainly of styrene, or the like, which are in contact with and sandwich the rubber layer and intermediate layers composed of a diene monomer and a hard resin layer component which are provided between the rubber layer and the inner resin layer and between the rubber layer and the outer resin layer.
2. Description of the Prior Art
Polystyrenes have been widely used as general-purpose resins because of their excellent transparency, water resistance and molding workability. However, because of their brittleness, which is one of the defects of polystyrenes, they have seldom been used in the preparation of films, sheets and the like.
Various methods have been attempted to overcome the brittleness of polystyrenes. In fact, styrene resins are available commercially for use in the manufacture of films and sheets which are high impact styrenes and there are biaxially oriented polystyrene films and sheets. However, the high impact styrenes are opaque because they are reinforced with rubber particles and are not suitable as wrapping films which must be transparent. The biaxially oriented films and sheets possess improved brittleness properties and also possess good transparency characteristics. However, they are still unsatisfactory as films or sheets because of their unacceptable bending strength, tear strength, and other properties. Moreover, involved preparative techniques and complicated equipment are required for the production of films and sheets from the available styrene materials.
U.S. Pat. No. 3,853,978 shows another attempt to employ polystyrenes for the fabrication of films and sheets which retain the good transparency characteristics of polystyrenes in which a styrene-butadiene tapered polymer is obtained by anionic polymerization of styrene thereby forming a transparent and high impact film or sheet. In this method polymers having a desired block structure are synthesized by strictly controlling the polymerization conditions of styrene and butadiene by a living anionic polymerization mechanism. That is, copolymers having a gradient composition are synthesized by gradually changing the styrene-butadiene chain mainly between styrene blocks and butadiene blocks. However, since this method is a complicated technique involving a living anionic polymerization mechanism, the monomers and solvents used must be of good quality and contain no oxygen or moisture. Furthermore, in this method the polymerization is effected in a hydrocarbon solvent such as benzene, n-hexane, or the like and the recovery of polymer from the polymerization solution must be accomplished by precipitating it in methanol, then isolating the polymer and vacuum drying it. Thus, the method requires extremely complicated steps and substantial investments in industrial equipment in comparison to the requirements of suspension polymerization and emulsion polymerization procedures which are used for the polymerization of ordinary sytrene resins.
In other approaches, suspension and emulsion polymerization procedures and the like, have been employed for the preparation of ABS resins for a long time. For example, in one method a hard resin layer composed mainly of styrene is provided as the nucleus of a diene elastic polymer in the preparation of a graft polymer in order to improve the impact resistance of the product prepared. The objective of all of these methods is to improve the impact strength of the product and hence a diene polymer which has a low Tg value is used as the nucleus and a monomer composed mainly of styrene or a mixture of styrene and acrylonitrile is added thereon and the polymerization reaction is conducted. Therefore, the resultant polymer possesses improved molding workability, but the excellent transparency inherent in polystyrene is lost and the molded product becomes opaque or translucent. Moreover, sheets of the product exhibit the so-called stress-whitening deficiency when they are bent and thus the product has substantial limitations when put to practical use.
It has further been proposed to form a layered product in which a hard resin such as styrene, methyl methacrylate, or the like is present in the nucleus of the product. U.S. Pat. No. 3,793,402 discloses a method of providing a high impact composition which has less haziness by forming a three-layer structure comprising a nucleus, an outer layer of resins of high Tg and an elastic polymer layer comprising a lower alkyl acrylate or butadiene between the nucleus and the outer layer or a multi-layer structure comprising repetitions of the three-layer structure while keeping the order of a hard resin.fwdarw.soft resin.fwdarw.hard resin and then blending the structure with polystyrene, polymethyl methacrylate, or the like. However, this composition has the defect common to many polymer blends such that when it is molded into a film, sheet or the like, whitening is apt to occur upon bending. Moreover, the transparency of the product is diminished in comparison to polystyrene or polymethyl methacrylate. In the multi-layer polymer per se, each layer is merely present as a block and consequently, the molded product is opaque or translucent. Furthermore, when the polymer product is molded into a film, limited molding conditions are operative and thus the polymer, in effect, can hardly be used as a suitable material for the preparation of films.
A need, therefore, continues to exist for a method of preparing styrene films and sheets and for general molding materials which possess excellent transparency properties and impact resistance properties and which do not exhibit stress whitening when molded into films or sheets and which are produced by an emulsion polymerization procedure which is advantageous from an industrial viewpoint.